1. Field of the Invention
This invention relates to thermocouple construction, particularly the construction of the hot junction of the thermocouple.
2. Description of the Prior Art
In prior art construction of thermocouples for use with gas appliances, a thermocouple wire is positioned within a tip tube with the ends of the wire and tube being fused to form the hot junction. Because the internal thermocouple wire is basically a nickel-containing alloy, such as an alloy of copper and nickel, and the external tip tube is generally an alloy of iron and chromium, the metallurgical properties of the fused heterogeneous metals forming the hot junction are not the most desirable for this purpose. In many instances, the interface between the fused alloys contains minute holes and gas pockets. If the joint is not properly annealed, high joint stresses result. These conditions cause accelerated thermal deterioration and short service life. Thermal corrosion of the thermocouple wire also may occur in the vicinity of the hot junction where the temperatures and stresses are highest. Further, in such thermocouples, because the internal wire is composed of a nickel-containing alloy, there is the nickel present at the surface of the fused joint. When the hot junction is inserted into a flame, such as the pilot of a gas appliance, the exposed nickel functions as a catalyst in cracking the natural gas fuel, producing carbon and carbon monoxide. Such carboning of the thermocouple is very undesirable in gas appliance uses.
It is accordingly a particular object of the present invention to provide a novel improved hot junction construction for a thermocouple, having improved properties such as substantial freedom from internal porous imperfections and stresses, providing a thermocouple of increased durability and longer life, yet which can be easily and economically fabricated by relatively simple procedure.